Method and apparatus for seamless handover in a wireless communication network

ABSTRACT

In a wireless communication network where base stations receive protocol data units (PDUs) from mobile stations for decompression and deciphering for ordered, sequential transfer as service data units (SDUs) to an associated core network, the teachings presented herein provide a method of supporting seamless handover of a mobile station from a source base station to a target base station. By way of example, the teachings herein apply to a network based on the E-UTRA specifications, as promulgated by the 3GPP. However, that example is non-limiting, as the teachings herein apply to any network that employs in-sequence data delivery and duplicate data detection at handover. Broadly, the source base station forwards out-of-sequence SDUs and corresponding sequence number information to the target base station in support of seamless handover, and the target base station uses that information to request retransmissions as needed for packet reordering.

This Application is a continuation of U.S. application Ser. No.12/596,692, which was the National Stage of International ApplicationNo. PCT/SE2008/050468, filed 23 Apr. 2008, which claims priority toSwedish patent application 0701011-9, filed 25 Apr. 2007, thedisclosures of all of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention generally relates to wireless communicationnetworks, and particularly relates to seamless handover in suchnetworks.

BACKGROUND

In a cellular system, mobile terminals are handed over between basestations. It is preferable that the change of the base-station occursseamlessly for the user. In particular, seamless handover minimizes orat least reduces interruption time, and prevents data losses andsequencing issues.

The Long Term Evolution (LTE) System currently standardized in 3GPP—seeTS 36.300 v8.0.0, 3GPP; Technical Specification Group Radio AccessNetwork; Evolved Universal Terrestrial Radio Access (E-UTRA) and EvolvedUniversal Terrestrial Radio Access Network (E-UTRAN); Overalldescription; Stage 2, March 2007—provides for movement of network usersbetween cells. More particularly, the specification includes Layer 2(L2) provisions related to providing seamless handover. These L2protocols include the Medium Access Control (MAC) protocol (with HybridAutomatic Repeat Request (HARQ) provisions), the Radio Link Control(RLC) protocol (with ARQ provisions), and the Packet Data ConvergenceProtocol (PDCP).

In a known implementation, a (core) network gateway or other packet dataentity delivers PDCP service data units to a base station for downlinkdelivery to a mobile station. The base station processes these servicedata units, such as by applying header compression and base-stationspecific ciphering, to obtain PDCP protocol data units that aretransmitted in sequence over the air interface to the mobile station.The mobile station receives these PDCP protocol data units and processesthem to recover the corresponding PDCP service data units. Notably, theprotocol data units include sequence number information, which allowsthe mobile station to detect missed protocol data units andcorrespondingly reorder data as needed to ensure that the mobile stationprocesses the service data units in their proper sequence.

Conversely, for uplink transmissions from the mobile station to the basestation, the mobile station processes PDCP service data unitsoriginating at the mobile station, to obtain corresponding PDCP protocoldata units, which are sequentially transmitted over the air interface tothe base station. The base station processes these received PDCPprotocol data units to obtain the corresponding PDCP service data units,and uses the sequence numbers to detect missing data and to conductreordering as needed for proper sequential transfer of the uplink PDCPservice data units to the core network.

Reception errors at the base station (and mobile station) can result inout-of-sequence reception. For example, the base station may receive twoPDCP protocol data units in sequence, miss a third one, and successfullyreceive a fourth one. That fourth one cannot be processed for transferto the core network until the missing third PDCP protocol data unit issuccessfully received at the base station. The base station thus buffersout-of-sequence data, holding it for reordering upon subsequent receiptof the missing data, which may be retransmitted according to knownARQ/HARQ processes in the MAC and RLC layers. The possibility ofout-of-sequence data reception adds potentially significantcomplications to mobile station handover, where the mobile station ishanded over from its current base station to a new base station.

SUMMARY

In a wireless communication network where base stations receive protocoldata units from mobile stations for ordered, sequential transfer asservice data units to an associated core network, the teachingspresented herein provide a method of supporting seamless handover of amobile station from a source base station to a target base station. Byway of example, the teachings herein apply to a wireless communicationnetwork based on the Evolved Universal Terrestrial Radio Access (E-UTRA)specifications, as promulgated by the Third Generation PartnershipProject (3GPP). For example, the protocol data units may comprise PacketData Convergence Protocol (PDCP) protocol data units and the servicedata units may comprise PDCP service data units. However, such examplesshould be understood as non-limiting, as the teachings hereinessentially apply to any wireless communication network that employsin-sequence data delivery and duplicate data detection at handover, e.g.at radio base stations within the radio access portion of the network.

In one embodiment, the teachings herein provide a method of handoverprocessing at a target base station, where the method supports seamlesshandover of a mobile station from a source base station to the targetbase station. In at least one such embodiment, the method comprises,during handover execution, receiving at the target base station servicedata units and sequence number information forwarded from the sourcebase station. The forwarded service data units are those service dataunits being held at the source base station for sequential transfer tothe associated core network, i.e., out-of-sequence service data units,and the forwarded sequence number information indicates the sequencenumbers corresponding to the forwarded service data units. As used here,sequence numbers corresponding to service data units simply are thesequence numbers of the protocol data units corresponding to thoseservice data units.

The method continues with reordering the forwarded service data units asneeded at the target base station for sequential transfer from thetarget base station to the associated core network. In this manner,re-sequencing functions are contained within the radio access portion ofthe wireless communication network, e.g., at the base station, and it isnot necessary for the associated core network to have re-sequencingcapabilities.

Such protocol and service data units may comprise, for example, PDCPprotocol and service data units. In at least one such embodiment, thesource base station deciphers and decompresses (if header compression isused) protocol data units incoming from a mobile station, to obtaincorresponding service data units for in-sequence transfer to theassociated core network. If protocol data units are received out ofsequence at the source base station, the corresponding service dataunits are held there for re-sequencing. Such buffered service data unitsare forwarded to a target base station, if handover to the target basestation is initiated for the corresponding mobile station.

Thus, in at least one embodiment, the teachings herein provide a methodof handover processing at a source base station, where the methodsupports seamless handover of a mobile station from the source basestation to a target base station. The method is characterized by, duringhandover execution, forwarding from the source base station to thetarget base station service data units and sequence number information.The forwarded service data units are those service data units being heldat the source base station for sequential transfer to the associatedcore network, and the forwarded sequence number information correspondsto the forwarded service data units. In this manner, the service dataunits being held at the source base station because of out-of-sequenceprotocol data unit receipt are forwarded to the target base station insupport of handover execution, along with an indication of the sequencenumbers of the corresponding protocol data units.

Further, the forwarded sequence number information includes in at leastone embodiment an indication of the sequence number for the protocoldata unit corresponding to the service data unit most recentlytransferred from the source base station to the associated core network.Additionally or alternatively, the forwarded sequence number informationcan include sequence number corresponding to the next service data unitexpected to be delivered in-sequence from the source base station to theassociated core network. As such, the target base station canconveniently process the forwarded sequence number information toidentify missing service data units.

A mobile station is also provided according to the teachings presentedherein. In one embodiment, a mobile station is configured for use in awireless communication network wherein the mobile station transmitsprotocol data units to a base station and the base station transferscorresponding service data units to an associated core network. Themobile station configured for supporting seamless handover from a sourcebase station to a target base station, and is characterized by ahandover processor. The handover processor of the mobile station isoperative to regenerate new protocol data units for missing service dataunits and transmit the regenerated protocol data units responsive tosignaling from the target base station. The regenerated protocol dataunits are regenerated from corresponding service data units buffered atthe mobile station and the (target base station) signaling indicateswhich service data units are missing at the target base station orequivalently indicates which service data units have been successfullyreceived at the target base station. For example, such signaling mayinclude explicit retransmission requests, and/or may include thetransmission of a status or other message from the target base stationthat indicates which service data units have been successfully receivedat the target base station, or which ones are missing at the target basestation.

Of course, the present invention is not limited to the above summary offeatures and advantages. Indeed, those skilled in the art will recognizeadditional features and advantages upon reading the following detaileddescription, and upon viewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of one embodiment of a wireless communicationnetwork, including source and target base stations configured accordingto an embodiment of seamless handover as taught herein.

FIG. 2 is a block diagram of functional processing associated PacketData Convergence Protocol (PDCP) processing.

FIG. 3 is a block diagram of one embodiment of a source and/or targetbase station, according to seamless handover teachings taught herein.

FIG. 4 is a block diagram illustrating an example of protocol andservice data unit processing and related statuses before, during, andafter handover execution according to seamless handover as taughtherein.

DETAILED DESCRIPTION

FIG. 1 illustrates a wireless communication network 10 that may be, butis not limited to, an E-UTRAN network that is configured according tothe relevant 3GPP specifications. The network 10 includes a radio accessnetwork (RAN) 12, and an associated core network 14. The RAN 12 includesa number of radio base stations 16, which may be, for example, “enhancedNode Bs,” also referred to as eNBs. The core network 14 includes apacket data gateway 18, such as a “System Architecture Evolution (SAE)Gateway (GW).”

With emphasis on processing features and configurations related toseamless handover of a mobile station 20 from one base station 16 toanother, FIG. 1 illustrates one base station 16 playing the role of“source base station,” and another base station 16 playing the role of“target base station.” For ease of discussion, the source base station16 is denoted as source base station 16-1 and the target base station 16is denoted as target base station 16-2.

Focusing particularly on uplink communications flowing from the mobilestation 20 to the gateway 18 via the base stations 16, one sees that themobile station 20 includes a Packet Data Convergence Protocol (PDCP)processor 32, and that the base stations 16 include corresponding PDCPprocessors 30. Assuming that the source base station 16-1 currently issupporting uplink communications from the mobile station 20, the mobilestation 20 transmits packet data to the gateway 18 based on forming PDCPprotocol data units—often referred to as “protocol data units”—based onciphering and (optionally) compressing PDCP service data units—oftenreferred to as “service data units”. For example, uplink packet data tobe transmitted from the mobile station 20 to the gateway 18 is formattedinto PDCP service data units, which are then processed into PDCPprotocol data units for over-the-air transmission to the source basestation 16-1.

In turn, the source base station 16-1 processes the received PDCPprotocol data units to obtain corresponding PDCP service data units,which it then transfers to the gateway 18. Because the PDCP service dataunits should be transferred to the gateway in order of transmissionsequence, the PDCP uplink processing includes assigning sequence numbersto the transmitted PDCP protocol data units. In this manner, the sourcebase station 16-1 may identify PDCP protocol data units that arereceived out of sequence at the source base station 16-1. The sourcebase station 16-1 may temporarily buffer the PDCP Service data unitscorresponding to the out-of-sequence PDCP protocol data units. Amongother advantages, the teachings presented herein provide advantageoussource and target base station processing wherein such buffered PDCPservice data units are forwarded from the source base station 16-1 tothe target 16-2 in support of seamless handover of the mobile station 20from the source base station 16-1 to the target base station 16-2.

More particularly, according to one or more embodiments taught herein,an interface 24 communicatively couples the source base station 16-1 tothe target base station 16-2, and that interface 24 is used by thesource base station 16-1 to forward service data units and correspondingsequence number information to the target base station 16-2 duringhandover execution. These forwarded service data units are identified byreference number “26” and the sequence number information is identifiedby reference number “28” in FIG. 1. The forwarded service data units 26are those service data units that correspond to protocol data unitsreceived out of sequence at the source base station 16-1 and that havenot been transferred to the gateway 18. In other words, the forwardedservice data units 26 are those service data units being held at thesource base station 16-1 for sequential transfer to the gateway 18.Correspondingly, the sequence number information 28 indicates at leastsequence numbers corresponding to the forwarded service data units. Inother words, the forwarded sequence number information indicates thesequence numbers of the out-of-sequence protocol data unitscorresponding to the forwarded service data units 26. Alternatively, thesequence number information 28 may include sequence numberscorresponding to the missing service data units.

Thus, as taught herein in one or more embodiments, a method ofsupporting seamless handover of a mobile station 20 from a source basestation 16-1 to a target base station 16-2 is characterized by, duringhandover execution, receiving at the target base station 16-2 servicedata units and sequence number information forwarded from the sourcebase station 16-1. The forwarded service data units comprise servicedata units being held at the source base station 16-1 for sequentialtransfer to the associated core network 14 and the forwarded sequencenumber information indicates sequence numbers corresponding to theforwarded service data units. The method continues with reordering theforwarded service data units as needed at the target base station 16-2for sequential transfer from the target base station 16-2 to theassociated core network 14.

In at least one embodiment, reordering the forwarded service data unitsas needed at the target base station 16-2 for sequential transfer fromthe target base station 16-2 to the associated core network 14 includesthe target base station 16-2 identifying missing service data unitsbased on the forwarded sequence number information, and requestingretransmission by the mobile station 20 of the protocol data unitscorresponding to the missing service data units. In at least one suchembodiment, the method is further characterized by the target basestation 16-2 receiving the retransmitted protocol data units andprocessing them to obtain the missing service data units, and reorderingthe thus obtained service data units with the forwarded service dataunits as needed for sequential transfer to the associated core network14.

The method is further characterized in one or more embodiments in thatthe target base station 16-2 processes the forwarded sequence numberinformation to identify protocol data units received at the target basestation 16-2 from the mobile station 20 that are duplicative with regardto service data units previously transferred from the source basestation 16-1 to the associated core network 14. The target base station16-2 also processes the forwarded sequence number information toidentify protocol data units for which retransmission by the mobilestation is required for reordering of the forwarded service data unitsfor sequential transfer from the target base station 16-2 to theassociated core network 14.

In at least one embodiment, forwarded service data units aresuccessfully deciphered versions of corresponding protocol data unitsthat were received out of sequence at the source base station 16-1 at orbefore initiation of handover and have not been transferred as servicedata units from the source base station 16-1 to the associated corenetwork 14. Note that in such embodiments, or in at least one otherembodiment, the forwarded service data units are successfully decipheredand decompressed versions of corresponding protocol data units that werereceived out of sequence at the source base station 16-1 at or beforeinitiation of handover and have not been transferred as service dataunits from the source base station 16-1 to the associated core network14. Note, too, that in at least one embodiment of the method, theforwarded sequence number information indicates the sequence numbercorresponding to the service data unit most recently transferred fromthe source base station 16-1 to the associated core network 14, orindicates the sequence number corresponding to a last in sequencereceived service data unit, or indicates the sequence numbercorresponding to the service data unit next expected to be transferredfrom the source base station 16-1 to the associated core network 14.(The “last in sequence received service data unit” denotes the lastservice data unit received in sequence from the mobile station 20 by wayof receiving uplink protocol data units from the mobile station 20.)

In any case, to better understand protocol and service data units usingPDCP as an example, FIG. 2 illustrates that each base station 16includes a PDCP processor 30, which may be a functional processingelement implemented in software, hardware, or any combination thereof.(The mobile station 20 includes a like PDCP processor 32.) One sees thatfunctionally the PDCP processor 30 forms downlink PDCP protocol dataunits for over-the-air transmission to the mobile station 20 by applyingciphering and (header) compression to PDCP service data units incomingfrom the gateway 18. Notably, the PDCP processor 30 alsogenerates/assigns a sequence number to each PDCP protocol data unit toenable sequential ordering and processing of PDCP service data units atthe mobile station 20. The PDCP processor 32 at the mobile station 20thus receives downlink PDCP protocol data units from a base station 16,and decompresses/deciphers them to obtain corresponding PDCP servicedata units. The receiver operation also includes the removal of sequencenumber information.

Similarly, one sees that functionally the PDCP processor 32 forms uplinkPDCP protocol data units for over-the-air transmission to a base station16 by applying ciphering and optionally applying header compression toPDCP service data units representing desired transmit information.Notably, the PDCP processor 32 also generates/assigns a sequence numberto each PDCP protocol data unit to enable sequential ordering andprocessing of PDCP service data units at the base station 16. The basestation 16 thus receives uplink PDCP protocol data units from the mobilestation 20, and its PDCP processor 30 decompresses/deciphers them toobtain corresponding PDCP service data units, where the sequencenumbering has been removed. For further details and examples, one mayrefer to TS 36.323, version 8.0.0, as released by the 3GPP.

Of particular interest with respect to PDCP processing, the basestations 16-1 and 16-2 are configured to support seamless handover ofthe mobile station 20. In general, it will be understood by thoseskilled in the art that the base stations 16 represent complexcomputation platforms that provide a comprehensive set of communicationcontrol and call processing functions and that such systems are subjectto wide variation in terms of their implementation. FIG. 3 thus will beunderstood as a non-limiting example of a functional implementation of abase station 16 in support of seamless handover processing.

In FIG. 3, the base station 16-x (where “x” denotes “1” or “2” withrespect to FIG. 1) includes a number of functional circuits orsub-systems, including a gateway interface circuit 40, a base stationinterface 42, which may be considered to be part of the logicalinter-base-station interface 24 introduced in FIG. 1, a wirelesscommunication interface 44 for wireless communications with the mobilestation 20 (and other mobile stations), and control/processing circuits46 for carrying out communication control and processing, and forcarrying on general base station operations.

The control/processing circuits 46 are, for example,microprocessor-based circuits, which may operate according to one ormore computer programs stored in a computer-readable medium within thebase station 16-x. Of course, it should be understood that thecontrol/processing circuits 46 may comprise hardware, software, or anycombination thereof. With that in mind, the control/processing circuits46 include a handover processor 48, along with the earlier-illustratedPDCP processor 30, which, again, may be implemented in software,hardware, or any combination thereof. Those skilled in the art willrecognize that the PDCP processor 30 may comprise part of a largerlayered protocol stack, and that its operations are coordinated withRadio Link Control (RLC) and Medium Access Control (MAC) processing.

The base station 16-x selectively operates as a source base station,e.g., 16-1 in FIG. 1, and as a target base station, e.g., 16-2 inFIG. 1. Referring again to reference numbers in FIG. 1, and with regardto target base station operation, the base station 16-2 implements amethod of supporting seamless handover of mobile stations 20 from thesource base station 16-1. In one or more embodiments, the method ischaracterized by, during handover execution, receiving at the targetbase station 16-2 PDCP service data units 26 and sequence numberinformation 28 forwarded from the source base station 16-1, where theforwarded service data units 26 and sequence number information 28correspond to service data units being held at the source base station16-1 for sequential transfer to the core network 14, and where theforwarded sequence number information indicates the sequence numberscorresponding to the forwarded service data units. The method is furthercharacterized by reordering the forwarded PDCP service data units asneeded at the target base station 16-2 for sequential transfer from thetarget base station to the associated core network.

In at least one embodiment, reordering the forwarded PDCP service dataunits as needed at the target base station 16-2 for sequential transferfrom the target base station 16-2 to the associated core network 14includes the target base station 16-2 identifying missing service dataunits based on the forwarded sequence number information 28, andrequesting retransmission by the mobile station 20 of the protocol dataunits corresponding to the missing service data units. The method may befurther characterized in that the forwarded sequence number information28 indicates sequence numbers corresponding to the forwarded servicedata units, and further indicates a sequence number corresponding to thePDCP service data unit most recently transferred from the source basestation 16-1 to the associated core network 14. The sequence numberinformation 28 may also include sequence numbers of the service dataunits that are missing at the source base station 16-1.

Such information may be transmitted in the form of a reception statusmessage sent from the source base station 16-1 to the target basestation 16-2 during handover execution. In at least one embodiment, theforwarded sequence number information 28 further indicates a sequencenumber of the PDCP protocol data unit corresponding to the PDCP servicedata unit most recently transferred from the source base station 16-1 tothe associated core network 14. That is, the forwarded sequence numberinformation may indicate sequence numbers of the PDCP protocol dataunits corresponding to the forwarded PDCP service data units 26, and mayfurther indicate the highest sequence number corresponding to PDCPservice data units already transferred in proper sequence from thesource base station 16-1 to the associated core network 14. As notedearlier, instead of indicating the sequence number corresponding to theservice data unit most recently transferred from the source base station16-1 to the associated core network 14, the forwarded sequence numberinformation may indicate the next higher sequence number.

The method may be further characterized in that the target base station16-2 processes the forwarded sequence number information 28 to identifyPDCP protocol data units received at the target base station 16-2 fromthe mobile station 20 that are duplicative with regard to PDCP servicedata units previously transferred from the source base station 16-1 tothe associated core network 14, and processes the forwarded sequencenumber information 28 to identify PDCP protocol data units for whichretransmission by the mobile station 20 is required for reordering ofthe forwarded PDCP service data units 26, for sequential transfer fromthe target base station 16-2 to the associated core network 14, e.g.,the gateway 18.

The method may be further characterized in that the forwarded PDCPservice data units 26 are successfully deciphered versions ofcorresponding PDCP protocol data units that were received out ofsequence at the source base station 16-1 at or before initiation ofhandover and have not been transferred from the source base station 16-1to the associated core network 14. If header compression is used, thenthe forwarded service data units may be decompressed. In this regard,then, it is not necessary for the target base station 16-2 to haveknowledge of ciphering keys in use at the source base station 16-1, noris it necessary for the source base station 16-1 to transfer headercompression state information to the target base station 16-2.

The complementary seamless handover method at the source base station16-1 is characterized by, during handover execution, forwarding from thesource base station 16-1 to the target base station 16-2 PDCP servicedata units 26 and sequence number information 28 corresponding to PDCPprotocol data units received out of sequence at the source base station16-1 and not yet transferred by the source base station 16-1 to theassociated core network 14. In other words, the source base station 16-1forwards the service data units it is holding for sequential transfer.As noted, such transfer may be further characterized in that theforwarded PDCP service data units 26 are successfully decipheredversions of the PDCP protocol data units received out of sequence at thesource base station 16-1 and not yet transferred by the source basestation 16-1 to the associated core network 14. Correspondingly, theforwarded sequence number information 28 indicates the sequence numbersof such out-of-sequence PDCP protocol data units.

Still further, the seamless handover method as implemented at the sourcebase station 16-1 may be characterized in that the source base station16-1 further includes in the forwarded sequence number information 28 anindication of the sequence number for the PDCP protocol data unitcorresponding to the PDCP service data unit most recently transferredfrom the source base station 16-1 to the associated core network 14, oran indication of the sequence number corresponding to the last insequence received service data unit. Alternatively, it may transfer thesequence number corresponding to the service data unit next expected tobe transferred from the source base station 16-1 to the associated corenetwork 14.

Thus, according to the example illustration of FIG. 3, seamlesssource-to-target handover is supported in one or more embodiments byconfiguring a target base station 16-2 to include a (base station)interface 42 and a handover processor 48. In at least one suchembodiment, the interface 42 is operative to receive PDCP service dataunits 26 and sequence number information 28 forwarded from the sourcebase station 16-1, where the forwarded service data units 26 are thoseservice data units being held at the source base station 16-1 forsequential transfer to the associated core network 14, and the forwardedsequence number information indicates corresponding sequence numbers forthe forwarded service data units. Correspondingly, the handoverprocessor 48 is operative to reorder the forwarded PDCP service dataunits as needed at the target base station 16-2, for sequential transferfrom the target base station 16-2 to the associated core network 14.

For complementary source base station operations, the handover processor48 and the associated interface 42 provide for forwarding from thesource base station 16-1 to the target base station 16-2 PDCP servicedata units 26 and sequence number information 28. As noted, theforwarded PDCP service data units 26 and sequence number information 28correspond to PDCP protocol data units received out of sequence at thesource base station 16-1 and not yet transferred by the source basestation 16-1 to the associated core network 14.

Of course, the mobile station 20 also includes one or more processingcircuits, e.g., a microprocessor-based system that implements the PDCPprocessor 32 shown in FIG. 1, along with being configured to supportseamless handover from the source base station 16-1 to the target basestation 16-2. In at least one embodiment, the mobile station 20 includesone or more processing circuits that are characterized by beingoperative to retransmit PDCP protocol data units for the missing PDCPservice data units, responsive to retransmission requests received fromthe target base station, and using new ciphering and header compressionstates to regenerate the PDCP protocol data units for retransmissionusing corresponding PDCP service data units buffered at the mobilestation 20. In this manner, the target base station 16-2 uses theforwarded sequence number information 28 at least in part to identifymissing service data units for which protocol data units retransmissionby the mobile station 20 is required. In this sense, theretransmission(s) are required so that the handover processor 48 in thetarget base station 16-2 can reorder the forwarded PDCP service dataunits 26, and transfer them to the associated core network 14 in thecorrect sequence.

Further, in at least one embodiment, the mobile station 20 is configuredfor supporting seamless handover from a source base station 16-1 to atarget base station 16-2, and is characterized by a handover processorthat is operative to regenerate new protocol data units for missingservice data units and transmit the regenerated protocol data unitsresponsive to signaling from the target base station 16-2 that indicateswhich service data units are missing at the target base station 16-2, orequivalently indicates which service data units have been successfullyreceived at the target base station 16-2.

The regenerated protocol data units are regenerated from correspondingservice data units buffered at the mobile station 20. In at least onesuch embodiment, the mobile station 20 is further characterized in thatthe regenerated protocol data units are regenerated using at least oneof new ciphering states and new header compression states. Further, inat least one embodiment, the mobile station 20 is further characterizedin that the mobile station 20 forgoes retransmission of those servicedata units that are indicated by the target base station 16-2 as havingbeen successfully received.

With the above in mind, then, those skilled in the art will appreciatethat the teachings herein include, in one or more embodiments, sendinginformation, including forwarded service data units, from a first basestation, e.g., source base station 16-1, to a second base station, e.g.,target base station 16-2, for seamless handover of a mobile station 20.Here, the base stations 16 and the mobile station 20 representrespective end-points of a protocol controlled link, e.g., a PDCP link.In at least one aspect, the teachings herein address service data unitreordering at the target base station 16-2 based on sending information,e.g., a status message, from the source base station 16-1 to the targetbase station 16-2. In at least one embodiment, the message includesinformation about the service data units or protocol data units thathave been successfully received at the source base station 16-1. Thisinformation can be used in the target base station 16-2 to request theretransmission of missing service data units after the handover iscompleted. The same status information can be used for duplicatedetection in the target base station 16-2, in case the target basestation 16-2 receives data that has already been delivered by the sourcebase station 16-1 to the gateway 18.

In at least one embodiment, uplink mobility as part of seamless handovercomprises sending all cumulatively correctly received data—i.e., allsequentially received or successfully reordered PDCP service dataunits—is sent via an “S1 interface” to gateway 18 in the associated corenetwork 14. The gateway 18 comprises, in an E-UTRAN embodiment, a SystemArchitecture Evolution (SAE) Gateway (GW).

Upon initiation of handover, the source base station 16-1 sends a statusmessage to the target base station 16-2 via the logical interface 26connecting these two base stations. (In LTE, this interface is named the“X2” interface. (In case the source and target base stations are notlogically interconnected with the “X2” interface, then theaforementioned communication including data forwarding and statusmessage transfer may be relayed via the core network 14, i.e., by usingthe two logical S1 interfaces between the two base stations 16 and thecore network 14.) The status message describes the reception status ofservice data units in the source base station 16-1. Note, that thestatus message does not necessarily represent the status at the mobilestation 20, which is the data sender for uplink communications.

The status message, which may be embodied in or otherwise accompany theforwarded sequence number information 28 described earlier herein, mayinclude the status of which service data units have been forwarded tothe gateway 18 by the source base station 16-1 over the S1 interface.Further, as noted, the source base station 16-1 also forwards thoseservice data units 26 that have been received in the source base station16-1 but not yet delivered to the gateway 18, i.e., those service dataunits being held in the source base station 16-1 because they are notreceived in-sequence and are waiting reordering at the source basestation 16-1. As explained earlier, such service data units aretemporarily buffered at the source base station 16-1 rather than beingtransferred to the gateway 18.

Assuming these buffered service data units are forwarded from the sourcebase station 16-1 to the target base station 16-2, the mobile station 20retransmits protocol data units to the target base station 16-2 thatcorrespond to the missing service data units. The mobile station 20performs such retransmission by, for example, generating new PDCPprotocol data units for the missing PDCP service data units missing atthe target base station 16-2. Note that the mobile station 20 generallyretains service data units while awaiting acknowledgment for thecorrespondingly transmitted protocol data units, and these retainedcopies can be used for protocol data unit regeneration andretransmission.

The mobile station 20 also is, in one or more embodiments, configured touse new ciphering and header compression states for retransmitting themissing PDCP service data units. The information included in the statusmessage sent from the source base station 16-1 to the target basestation 16-2 can be used by the target base station 16-2 to requestretransmissions by the mobile station 20, using known ARQ mechanisms(e.g. in the RLC protocol).

The status message in one or more embodiments includes at least asequence number (SN) that indicates up to which SN the data has beencorrectly received at the source base station 16-1. The target basestation 16-2 can be configured to determine which service data units aremissing based on the sequence numbers corresponding to the forwardedservice data units 26. However, in at least one embodiment, the sourcebase station 16-1 explicitly indicates the missing data, such as in theform of a data list or bit-map. In addition to the ARQ relatedinformation, the status report may also include information on theprotocol data units that were (or were not) successfully decompressedand/or deciphered. Doing so allows the target base station 16-2 torequest retransmission of service data units that were correctlyreceived in the source in an ARQ sense at the source base station 16-1but were not successfully decompressed or deciphered, i.e. thecorresponding protocol data units were successfully received, but thesource base station 16-1 may have failed in processing service dataunits from the received protocol data units.

By way of non-limiting example, FIG. 4 “logically” illustrates aseamless handover of the mobile station 20 from the source base station16-1 to the target base station 16-2. The diagram is a “logical”illustration in the sense that it shows the logical condition oftransmit and receive statuses for PDCP service data units transmittedfrom the mobile station 20 to the source base station 16-1 and/or to thetarget base station 16-2. These illustrated status indicators may or maynot represent the literal data structures used in the mobile station 20and in the base stations 16 for managing PDCP-based uplinkcommunications, but rather are for purposes of discussion.

In FIG. 4, the source and target base stations 16-1 and 16-2,respectively, operate according to the teaching presented herein forseamless handover. For discussion purposes, it is assumed that themobile station 20 has transmitted a number of PDCP protocol data unitsto the source base station 16-1, with some of them received successfullyand some of them not. The illustrated status indicators 50 indicate thestatus at the mobile station 20 of those transmitted PDCP service dataunits, and they indicate that PDCP protocol data units having sequencenumbers 3-13 have been transmitted from the mobile station 20 to thesource base station 16-1 in advance of the handover.

However, as noted, not all of the PDCP protocol data units weresuccessfully received at the source base station 16-1, and the receivedstatus indicators 52 at the source base station 16-1 illustrate anexample case, wherein PDCP service data units have been successfullyreceived (including successful decompression/deciphering) for PDCPprotocol data units having sequence numbers 3, 4, 5, 6, 8, 11, and 12.Contrastingly, PDCP service data units have not been successfullyreceived for sequence numbers 7, 9, and 10. As such, the PDCP servicedata units 8, 11, and 12 are considered as having been receivedout-of-sequence. Also of note in FIG. 4 are the transferred PDCP servicedata units 54. These service data units 54 represent those PDCP servicedata units already transferred from the source base station 16-1 to theassociated core network 14 (e.g., gateway 18) before handover execution.

Thus, during handover execution, the source base station 16-1 forwardsthe PDCP service data units 26 that were received out of sequence andare awaiting reordering for transfer to the gateway 18. In this example,the forwarded PDCP service data units are 8, 11, and 12. That is, theforwarded PDCP service data units are those corresponding to the 8th,11th, and 12th PDCP protocol data units in the sequence, which weresuccessfully received at the source base station 16-1, but received outof sequence. As already noted, the source base station 16-1 forwards thesequence number information 28 corresponding to these forwarded PDCPservice data units, and, in one or more embodiments, the sequence numberinformation 26 includes an indication of the highest sequence number ofPDCP service data units already transferred up to the gateway 18—i.e.,the most recently sequentially transferred PDCP service data unit. Inthis example, that value is “6.” Different ways of coding thisinformation can be applied, e.g. where the most recently sequentiallytransferred PDCP service data unit may be indicated (“6”), oralternatively, the next PDCP service data unit to be sequentiallytransferred is indicated (“7”).

In complementary fashion, the target base station 16-2 uses the receivedsequence number information 26 to request selective retransmissions bythe mobile station 20, while avoiding unnecessary retransmissions andthereby improving handover efficiency. More particularly, the targetbase station 16-2 processes the received sequence number information 26,which may include or comprise the status message information discussedearlier herein, to identify the missing PDCP protocol data units thatthe mobile station 20 needs to retransmit to allow the target basestation 16-2 to properly sequence the forwarded PDCP service data units26 for transfer to the gateway 18.

Thus, the status indicators 56 represent the status of the mobilestation 20 sometime after handover, assuming that retransmissionrequests (e.g., ARQ procedures) at the target base station 16-2 haveprompted the mobile station 20 to retransmit the missing data. That is,one sees that the missing data for sequence numbers 7, 9, 10, and 13 hasbeen successfully retransmitted in the form of PDCP protocol data unitsfrom the mobile station 20 to the target base station 16-2. The PDCPservice data units 58 corresponding to the retransmitted PDCP protocoldata units are explicitly shown on the uplink to the target base station16-2. Note that hatching is used to indicate retransmitted data.

Continuing with the example, the received status indicators 60 at thetarget base station 16-2 illustrate that the target base station 16-2has obtained the missing data needed to properly reorder the forwardedPDCP service data units 26. Thus, the target base station 16-2 transfersthe retransmitted PDCP service data units and the forwarded PDCP servicedata units 26 in proper sequence order to the gateway 18. The set oftransferred PDCP service data units 62 indicates that transfer.

Of course, FIG. 4 stands as a representative example, and is notlimiting. Indeed, the present invention is not limited to the foregoingdiscussion and accompanying drawings. Instead, the present invention islimited only by the following claims and their legal equivalents.

What is claimed is:
 1. A method, in a wireless communication networkwhere base stations receive protocol data units from mobile stations forordered, sequential transfer as service data units to an associated corenetwork, of supporting seamless handover of a mobile station from asource base station to a target base station, the method comprising:during handover execution, receiving at the target base station servicedata units and sequence number information forwarded from the sourcebase station, wherein: the forwarded service data units comprise servicedata units being held at the source base station for sequential transferto the associated core network; and the forwarded sequence numberinformation indicates sequence numbers corresponding to the forwardedservice data units; reordering the forwarded service data units asneeded at the target base station for sequential transfer from thetarget base station to the associated core network; wherein thereordering the forwarded service data units comprises the target basestation: identifying missing service data units based on the forwardedsequence number information; and requesting retransmission, by themobile station, of protocol data units regenerated from the missingservice data units buffered at the mobile station.
 2. The method ofclaim 1 further comprising the target base station: receiving theretransmitted protocol data units and processing them to obtain themissing service data units; and reordering the thus obtained servicedata units with the forwarded service data units as needed forsequential transfer to the associated core network.
 3. The method ofclaim 1 wherein the forwarded sequence number information indicates thesequence numbers of the protocol data units corresponding to theforwarded service data units.
 4. The method of claim 1 wherein theforwarded sequence number information indicates the sequence numbercorresponding to the service data unit most recently transferred fromthe source base station to the associated core network, or indicates thesequence number corresponding to a last in sequence received servicedata unit.
 5. The method of claim 1 wherein the target base station:processes the forwarded sequence number information to identify protocoldata units received at the target base station from the mobile stationthat are duplicative with regard to service data units previouslytransferred from the source base station to the associated core network;and processes the forwarded sequence number information to identifyprotocol data units for which retransmission by the mobile station isrequired for reordering of the forwarded service data units forsequential transfer from the target base station to the associated corenetwork.
 6. The method of claim 1 wherein the forwarded service dataunits are successfully deciphered versions of corresponding protocoldata units that were received out of sequence at the source base stationat or before initiation of handover and have not been transferred asservice data units from the source base station to the associated corenetwork.
 7. The method of claim 1 wherein the forwarded service dataunits are successfully deciphered and decompressed versions ofcorresponding protocol data units that were received out of sequence atthe source base station at or before initiation of handover and have notbeen transferred as service data units from the source base station tothe associated core network.
 8. The method of claim 1: wherein theprotocol data units comprise Packet Data Convergence Protocol (PDCP)protocol data units; wherein the service data units comprise PDCPservice data units.
 9. A target base station supporting seamlesshandover of a mobile station from a source base station to the targetbase station, in a wireless communication network where base stationsreceive protocol data units from mobile stations for ordered, sequentialtransfer as service data units to an associated core network, the targetbase station comprising: an interface configured to receive service dataunits and sequence number information forwarded from the source basestation, wherein: the forwarded service data units comprise thoseservice data units being held at the source base station for sequentialtransfer to the associated core network; and the forwarded sequencenumber information indicates sequence numbers corresponding to theforwarded service data units; a processing circuit configured to reorderthe forwarded service data units as needed at the target base stationfor sequential transfer from the target base station to the associatedcore network by: identifying missing service data units based on theforwarded sequence number information; requesting retransmission by themobile station of protocol data units regenerated from the missingservice data units buffered at the mobile station.
 10. The target basestation of claim 9 wherein the target base station is configured to:receive the retransmitted protocol data units and process them to obtainthe missing service data units; and reorder the thus obtained servicedata units with the forwarded service data units as needed forsequential transfer to the associated core network.
 11. The target basestation of claim 9 wherein the sequence number information furtherindicates the sequence number corresponding to the service data unitmost recently transferred from the source base station to the associatedcore network, or indicates the sequence number corresponding to a lastin sequence received service data unit.
 12. The target base station ofclaim 9 wherein the processing circuit is configured to: process theforwarded sequence number information to identify protocol data unitsreceived at the target base station from the mobile station that areduplicative with regard to service data units previously transferredfrom the source base station to the associated core network; and processthe forwarded sequence number information to identify protocol dataunits for which retransmission by the mobile station is required forreordering of the forwarded service data units for sequential transferfrom the target base station to the associated core network.
 13. Thetarget base station of claim 9 wherein the forwarded service data unitsare successfully deciphered versions of corresponding protocol dataunits that were received out of sequence at the source base station ator before initiation of handover and have not been transferred asservice data units from the source base station to the associated corenetwork.
 14. The target base station of claim 9 wherein the forwardedservice data units are successfully deciphered and decompressed versionsof corresponding protocol data units that were received out of sequenceat the source base station at or before initiation of handover and havenot been transferred as service data units from the source base stationto the associated core network.
 15. A method of supporting seamlesshandover of a mobile station from a source base station to a target basestation, in a wireless communication network where base stations receiveprotocol data units from mobile stations for ordered, sequentialtransfer as service data units to an associated core network, the methodcomprising: forwarding from the source base station to the target basestation, during handover execution, service data units and sequencenumber information, wherein: the forwarded service data units comprisethose service data units being held at the source base station forsequential transfer to the associated core network; and the forwardedsequence number information indicate sequence numbers corresponding tothe forwarded service data units; wherein the mobile station, responsiveto signaling from the target base station that indicates which theforwarded service data units are missing at the target base station,regenerates new protocol data units corresponding to the missing servicedata units buffered at the mobile station and transmits the regeneratedprotocol data units to the target base station.
 16. The method of claim15 wherein the forwarded service data units are successfully decipheredversions of the corresponding protocol data units received out ofsequence at the source base station, such that the forwarded sequencenumber information indicates the sequence numbers of suchout-of-sequence protocol data units.
 17. The method of claim 15 whereinthe source base station: includes, in the forwarded sequence numberinformation, an indication of the sequence number corresponding to theservice data unit most recently transferred from the source base stationto the associated core network; or includes, in the forwarded sequencenumber information, an indication of the sequence number correspondingto a last in sequence received service data unit.
 18. A method in amobile station in a wireless communication network, wherein the mobilestation transmits protocol data units to a base station and the basestation transfers corresponding service data units to an associated corenetwork, the mobile station configured to support seamless handover froma source base station to a target base station, the method comprising:regenerating new protocol data units for missing service data units andtransmitting the regenerated protocol data units responsive to signalingfrom the target base station that indicates: which service data unitsare missing at the target base station; or indicates which service dataunits have been successfully received at the target base station; wherethe regenerated protocol data units are regenerated from correspondingservice data units buffered at the mobile station.
 19. The method ofclaim 18 further comprising generating the regenerated protocol dataunits using at least one of new ciphering states and new headercompression states.
 20. The mobile station of claim 18 furthercomprising forgoing retransmission of those service data units that areindicated by the target base station as having been successfullyreceived.